Multi-axle steering system for vehicles

ABSTRACT

A multi-axle steering system which is intended for vehicles has a steering gear (4) for a first steering axle (1) and hydrostatic steering for a second steering axle (2). A master cylinder (11) is disposed on the first steering axle (1) and a slave cylinder (23) on the second steering axle (2). The work chambers (12, 13; 24, 25) of the master cylinder (11) and the slave cylinder (23) are connected with each other by pressure lines (26, 27) and in this way together constitute the hydrostatic steering. A centering device (34) for a return of the steered wheels of the second steering axle (2) into their straight-ahead running position is provided on the second steering axle (2). A device (38) on the master cylinder (11) is used for automatic synchronization of the steered wheels of the first steering axle (1) and the second steering axle (2), by means of which a connection can be made only between the two work chambers (12, 13) of the master cylinder (11) in the straight-ahead running position and an adjacent small area.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a multi-axle steering system for vehicles. Thesteering system includes a steering gear for a first steering axle andhydrostatic steering for a second steering axle, wherein a mastercylinder is disposed on the first steering axle and a slave cylinder isdisposed on the the second steering axle, whose work chambers areconnected with each other by means of pressure lines and which togetherconstitute the hydrostatic steering, and wherein the second steeringaxle has a centering device to return of the steered wheels of thesecond steering axle into their straight-ahead running position.

2. Description of the Prior Art

This type of multi-axle steering system is known from European PatentPublication EP 0 150 520 B1. This multi-axle steering system is intendedfor a vehicle with three axles. The front axle and the second rear axleof these three axles are designed to be steerable. The known multi-axlesteering system has an automatic stabilizer for the straight-aheadrunning position of the rear steering axle. To stabilize thestraight-ahead running position of the rear steering axle, a fluidconnection with a reservoir is made at or near the center position ofthe front steering axle, by means of which pressure relief of ahydraulic cylinder disposed on the front axle is achieved when it is inthe center position. It is disadvantageous in connection with thismulti-axle steering system that after each passage through the centerposition it is necessary to build up the pressure from zero in thehydraulic cylinder acting as the master cylinder for the hydrostaticsteering.

It is the object of the invention to improve a multi-axle steeringsystem for vehicles of the known type in such a way that the mentioneddisadvantages are avoided, in particular in that pressure losses andtherefore energy losses are prevented as much as possible andsatisfactory synchronization of the two steering axles is assured.

SUMMARY OF THE INVENTION

This object is attained by the multi-axle steering system The attainmentis achieved in that a device for the automatic synchronization of themovements of the master cylinder and the slave cylinder is provided onthe master cylinder, by means of which a connection between only the twowork chambers of the master cylinder can be made in the straight-aheadrunning position and in a small area adjoining it.

Practical and advantageous embodiments of the invention are described inthe detailed description herein. However, the invention is not limitedto the combination of the specific characteristics of the preferredembodiments. Further useful combination options of the preferredembodiments and individual preferred embodiments result from the objectof the invention for one skilled in the art.

The device for synchronizing is formed in a particularly advantageousand simple manner in a guide insert of the piston rod of the mastercylinder by means of an elastic seal ring, which is maintained in anannular groove of the guide insert, which is radially open to theoutside. The seal ring acts together with a ring-shaped contraction ofthe piston rod in such a way that in the straight-ahead running positionand in an area adjoining it between the seal ring and the constriction,there is a connection between the work chambers of the master cylinder,and that outside of this area the seal ring rests sealingly against thecylindrical portion of the piston rod so that the two work chambers areseparated from each other in this way.

In this embodiment a direct connection between the two work chambers ofthe master cylinder and a reservoir is not possible in thestraight-ahead running position. There is only a connection between thetwo work chambers of the master cylinder, so that there can be nopressure build-up in the work chambers of the slave cylinder and thesecond steering axle remains centered in its straight-ahead runningposition. In this way the synchronization of the slave cylinder with themaster cylinder is assured during each passage of the master cylinderthrough the straight-ahead running position.

The length of the area of the straight-ahead running position, in whicha pressure build-up in the slave cylinder is intended to be prevented,can be easily established by the length of the constriction of thepiston rod, in that the length of its cylindrical area and/or the lengthand inclination of the transition to the full exterior diameter of thepiston rod are varied.

Two compensation valves are disposed between the two work chambers ofthe master cylinder or the slave cylinder, only one of which can beclosed at one time. The backs of the compensation cylinders facing awayfrom the work chambers are connected to a pressure reservoir. By meansof this it is achieved that the system pressure can be maintainedapproximately constant during temperature changes, i.e. for example nopressure increase takes place with a temperature increase. It isfurthermore possible to pre-stress the hydrostatic steering system bymeans of the pressure reservoir connected with it. The stiffness of thesystem is assisted by this.

It is particularly advantageous to design the two compensation valves asball seat valves having two valve seats facing away from each other andarranged coaxially in respect to each other. The distance between thevalve seats is fixed in such a way that the valve balls touch each otherbefore one of the ball valves rests on one of the valve seats. It isassured by means of this that only respectively one of the compensationvalves can be closed. The two compensation valves can be disposed in aparticularly space-saving manner in the guide insert.

Two pressure relief valves, which are advantageous for the two sides ofthe hydrostatic steering, can also be disposed in the guide insert.

If the centering device of the second steering axle is embodied as ahydraulic cylinder-piston unit which is connected with the pressurereservoir, this results in a simple manner in a centering device whosecentering power can be adjusted by means of the pressure of the pressurereservoir.

The invention will be explained in more detail in what follows by meansof two exemplary embodiments represented in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a schematic representation of a hydraulic circuitdiagram of the multi-axle steering system in accordance with theinvention in a first exemplary embodiment, with a master cylinder on afirst steering axle and a slave cylinder on a second steering axle,

FIG. 2 is a longitudinal section through a structural embodiment of themaster cylinder,

FIG. 3 is a longitudinal section through a guide insert of the mastercylinder in FIG. 2,

FIG. 4 is a longitudinal section through a structural embodiment of theslave cylinder, and

FIG. 5 is a hydraulic circuit diagram of the multi-axle steering systemin accordance with the invention in a second exemplary embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The multi-axle steering system in accordance with the invention isintended, for example, for a vehicle with three axles, of which a firststeering axle 1 is embodied as a steerable front axle of the vehicle,and a second steering axle 2 is arranged as a steerable trailing axlebehind a rigid rear axle 3 of the vehicle. The first steering axle 1 iscontrolled by a steering gear 4. The steering gear 4 is embodied as apower- assisted steering which is connected with a servo pump 5 and areservoir 6. The servo pump 5 is usefully driven by the vehicle engine7. The embodiment of the steering gear 4 is not essential for theinvention and will therefore not be described in detail. In place of apower-assisted steering it is also possible to provide manual steering,for example for smaller, lighter vehicles. With both power-assistedsteering and manual steering there is usefully a mechanical connectionfrom the steering gear 4 to the first steering axle 1 via a steeringlinkage 8.

A piston rod 10 of a master cylinder 11 is connected with the steeringlinkage 8. The master cylinder 11 contains two pressure chambers 12 and13, between which a guide insert 14 with a centered, essentiallycylindrical opening 15 for the piston rod 10 is located. If, as in FIG.1, the steering gear 4 is designed as a power-assisted steering, twofurther work chambers 16 and 17, which are used for the power assist ofthe steering movements of the first steering axle 1, follow the two workchambers 12 and 13 of the actual master cylinder 11. These two workchambers 16 and 17 are connected with the steering gear 4 via work lines18 and 20.

A piston rod 22 of a slave cylinder 23 is connected with a steeringlinkage 21 of the second steering axle 2. The slave cylinder 23 has twowork chambers 24 and 25, which are connected via lines 26 and 27 withthe work chambers 12 and 13 of the master cylinder 11. Together with thelines 26 and 27, the master cylinder 11 and the slave cylinder 23constitute a hydrostatic steering of the second steering axle 2.

The slave cylinder 23 contains two further work chambers 28 and 30,which are both connected with a pressure reservoir 31. Together with twopistons 32 and 33, which are floatingly seated in the slave cylinder,the two work chambers 28 and 30 constitute a centering device 34 for areturn of the steered wheels of the second steering axle 2 into thestraight-ahead running position. By means of the reservoir pressureexisting in the work chambers 28 and 30, the two pistons 32 and 33 pushthe piston rod 22 via a spring washer 35, fastened at its end, in thedirection toward a central stop formed by a spring washer 36 fixed inplace in the slave cylinder 23. An air-filled space 37 is locatedbetween the two pistons 32 and 33, in which a partial vacuum exists whenthe second steering axle 2 is deflected.

A device 38 for the automatic synchronization of the movements of themaster cylinder 11 and the slave cylinder 23 is provided on the mastercylinder 11, usefully in the guide insert 14. The device 38 is formed byan elastic seal ring 39 which is held in an annular groove 40 in theguide insert 14, radially open toward the interior in the directiontoward the opening 15. The seal ring 39 works together with aring-shaped constriction 41 of the piston rod 10. In its center theconstriction 41 has a cylindrical area, which is followed on both sidesby a transition area up to the full exterior diameter of the piston rod10. In the straight-ahead running position of the vehicle wheels of thefirst steering axle 1 which are to be steered, the constriction 41 islocated in the area of the seal ring 39. An annular gap exists betweenthe seal ring 39 and the cylindrical area of the constriction 41 in thestraight-ahead running position, through which, together with two axialgrooves 42 and 43, an open connection between the two work chambers 12and 13 of the master cylinder 11 is provided. The two axial grooves 42and 43 are formed in the interior of the opening 15 and are open towardboth sides of the guide insert 14. The transition from the cylindricalarea of the constriction 41 to the full exterior diameter of the pistonrod 10 usefully consists of faces of a truncated cone with a very smallinclination, so that, when the piston rod 10 is displaced, the seal ring39 can continuously expand until it finally rests sealingly on thecylindrical exterior of the piston rod 10 and during the furthermovement of the piston rod 10 completely blocks the connection betweenthe two work chambers 12 and 13 in this way.

The length of the effective constriction 41 determines the steeringangle of the first steering axle 1 within which the second steering axle2 remains hydraulically centered and does not also steer. The device 38for synchronization is sealed outside of the area of the constriction41. In this case the hydrostatic connection between the first steeringaxle 1 and the second steering axle 2 is active. This means thatadjustment forces in connection with higher pressures act on the secondsteering axle 2. If leaks in the hydrostatic system occur in this phase,the association of the slave cylinder 23 with the master cylinder 11 ischanged. This change is compensated every time the constriction 41 iscrossed, i.e. a synchronization is performed. In this embodiment of thedevice 38 it is important that the effective piston faces of the workchambers 12, 13 of the master cylinder 11 are of the same size. Unequalfaces would lead to a hydraulic blockage during synchronization. Theslave cylinder 23 also has equal piston surfaces.

To prevent the appearance of a pressure increase when the temperature ofthe multi-axis steering system rises, compensating valves 44 and 45 aredisposed between each work chamber of the master cylinder 11 or theslave cylinder 23 on the one side and the pressure reservoir 31 on theother side. In the schematic exemplary embodiment of FIG. 1, thecompensating valves 44 and 45 are disposed between the two lines 26 and27 on the one side and a line 46 leading to the pressure reservoir 31 onthe other side. The two compensating valves 44 and 45 are integratedinto the guide insert 14 in the structural embodiment of FIG. 3. In thiscase they are designed as ball seat valves and have two valve seats 47and 48, which face away from each other and are arranged coaxially inrespect to each other. The distance between the valve seats 47 and 48 isfixed in such a way that the valve balls touch before one of the valveballs comes to rest on one of the valve seats 47 and 48. A bore 50 islocated between the two valve seats 47 and 48, which leads to an annulargroove 51 with which the pressure reservoir 31 is connected. By means ofthe fixed distance between the two valve seats 47 and 48 it is achievedthat respectively only one work chamber 12 or 13 can be connected withthe pressure reservoir 31 via the bore 50, while the other work chamber13 or 12 is separated from the pressure reservoir 31. A connectionbetween the two work chambers 12 and 13 via the compensating valves 44and 45 is therefore also not possible.

To limit the working pressure in the hydrostatic steering, a pressurerelief valve 52 or 53 is provided for each side of this steering. In theexemplary embodiment of FIG. 1, these pressure relief valves 52 and 53have been inserted between the lines 26 and 27 on the one side and theline 46 leading to the pressure reservoir 31. In the exemplaryembodiment in FIG. 3, the pressure relief valves 52 and 53, togetherwith the compensating valves 44 and 45, are disposed in the guide insert14. Here, too, the side of the pressure relief valves 52 and 53 facingaway from the pressure is connected with the pressure reservoir 31 via abore 54 and the annular groove 51.

In the exemplary embodiment of FIG. 5 the compensating valves 44 and 45and the pressure reservoir 31 are disposed in the direct vicinity of theslave cylinder 23 of the second steering axle 2. Similar to theexemplary embodiment in FIG. 1, the two compensating valves 44 and 45have been inserted between the two lines 26 and 27 on the one side and aline 55 leading to the pressure reservoir 31 on the other side. Theadvantage of this arrangement lies in that the long line 46 from thefront to the back, which is required in the exemplary embodiment of FIG.1, can be omitted. The pressure relief valves 52 and 53 are arranged insuch a way that the side facing away from the pressure is respectivelyconnected with the other side of the hydrostatic steering, so that thecheck valve 53 connected to the line 26 opens into the line 27 and viceversa.

If the pressure of the pressure reservoir 31 is designed to beadjustable, the centering force of the centering device 34 can beadjusted by means of this.

What is claimed is:
 1. A multi-axle steering system for vehiclescomprising a steering gear for a first steering axle having steeredwheels and with hydrostatic steering for a second steering axle havingsteered wheels,wherein a master cylinder having two work chambers andbeing disposed on the first steering axle and a slave cylinder havingtwo work chambers and being disposed on the second steering axle, thefour work chambers being connected with each other by means of pressurelines and which elements together constitute the hydrostatic steering,wherein the second steering axle has a centering device for the returnof the steered wheels of the second steering axle to a straight-aheadrunning position, the improvement comprisingthe two work chambers of themaster cylinder are provided with equal effective surfaces, and a devicefor the automatic synchronization of the movement of the master cylinderand the slave cylinder is provided on the master cylinder, by means ofwhich device a connection between only the two work chambers of themaster cylinder can be made when the steered wheels of the firststeering axle are in or near the straight-ahead running position.
 2. Amulti-axle steering system in accordance with claim 1, wherein themaster cylinder includes a piston rod and the device for synchronizingthe movements of the master cylinder and the slave cylinder is formed ina guide insert of the piston rod of the master cylinder by an elasticseal ring which is maintained in an annular groove of the guide insert,the annular groove being radially open to the outside and which,together with a ring-shaped constriction of the piston rod, ensures thatwhen the steered wheels of the first steering axle are in thestraight-ahead running position there is an open connection between thework chambers of the master cylinder in an area between the seal ringand the constriction adjoining the seal ring; and that outside of thearea between the seal ring and constriction adjoining the seal ring, theseal ring rests in sealing engagement against a cylindrical portion ofthe piston rod such that the two work chambers of the master cylinderare not connected to one another.
 3. A multi-axle steering system inaccordance with claim 2, wherein the constriction of the piston rodincludes a central cylindrical area and adjacent the center cylindricalarea of the constriction of the piston rod, the piston rod graduallyincreases in diameter until it reaches a maximum external diameter.
 4. Amulti-axle steering system in accordance with claim 1, furthercomprising at least two compensating valves one of which is disposedbetween a work chamber of the master cylinder and a fluid-containingpressure reservoir, and the other of which is disposed between a workchamber of the slave cylinder and the pressure reservoir to provide afluid connection from the pressure reservoir to the respective workchamber via the compensating valve and wherein at least one work chamberof the master cylinder is not connected to the pressure reservoir.
 5. Amulti-axle steering system in accordance with claim 4, wherein the atleast two compensating valves are ball seat valves.
 6. A multi-axlesteering system in accordance with claim 5, wherein the compensatingvalves are disposed in the guide insert.
 7. A multi-axle steering systemin accordance with claim 6, wherein the compensating valves each havetwo valve seats, respectively, which valve seats face away from oneanother and are disposed coaxially with respect to one another and thedistance between the valve seats is fixed in such a way that the valveballs touch one another before one of the valve balls rests against oneof the valve seats.
 8. A multi-axle steering system in accordance withclaim 7, wherein a bore is provided between the two valve seats of eachof the compensating valves, and said bore leads to an annular groovelocated on an exterior circumference of the guide insert, and thepressure reservoir is fluidly connected to the annular groove.
 9. Amulti-axle steering system in accordance with claim 1, wherein thehydrostatic steering has two sides and each side of the hydrostaticsteering is provided with a pressure relief valve.
 10. A multi-axlesteering system in accordance with claim 9, wherein the pressure reliefvalves are disposed in a guide insert.
 11. A multi-axle steering systemin accordance with claim 10, wherein each pressure relief valve has twosides, one of which faces toward the pressure from the hydrostaticsteering and the other of which faces away from the pressure of thehydrostatic steering and is fluidly connected with the pressurereservoir.
 12. A multi-axle steering system in accordance with claim 11,wherein the centering device of the second steering axle is a hydrauliccylinder-piston unit and is fluidly connected to the pressure reservoir.13. A multi-axle steering system in accordance with claim 5, wherein thecompensating valves and the pressure reservoir are disposed in thevicinity of the slave cylinder of the second steering axle such that thecompensating valves are located between the pressure lines.
 14. Amulti-axle steering system in accordance with claim 12, wherein thecompensating valves and the pressure reservoir are disposed in thevicinity of the slave cylinder of the second steering axle such that thecompensating valves are located between the pressure lines.
 15. Amulti-axle steering system in accordance with claim 12, wherein thepressure of a fluid contained in the pressure reservoir is adjustable.